Up to the present, for thermally insulated containers which can be used as thermos bottles, thermally insulated lunch boxes, and the like, development of thermally insulated synthetic resin containers, which have the advantages of being light weight and easily molded, and having low manufacturing costs, have been progressing. As this type of thermally insulated synthetic resin containers, a container having a lid which can be fitted thereto having the following structure has been proposed. The thermally insulated container has an inner container and an outer container formed of synthetic resin. The inner container is combined with the outer container, which is dimensionally larger than the inner container, while leaving a space between the two containers. The inner container and the outer container are joined at the mouth portions to form a double wall structured container. A gas having a low thermal conductivity comprising at least one gas from a group consisting of xenon, krypton, and argon is filled in the space formed between the inner container and the outer container. In the same manner, the thermally insulated lid has a top surface wall and a bottom surface wall formed of synthetic resin. The top surface wall is combined with the bottom surface wall, while leaving a space between the two walls. The top surface wall and the bottom surface wall are joined at the edge portions to form a double wall structured lid. Preferably, gas having a low thermal conductivity comprising at least one gas from a group consisting of xenon, krypton, and argon is filled in the space formed between the top surface wall and the bottom surface wall.
As for these types of thermally insulated containers or lids, so as to improve the effects of the thermal insulation, in addition to the insertion of the gas having low thermal conductivity into the thermal insulation layer, a metallic foil is disposed at least on the surface of the side of the thermal insulation layer of the inner container or the bottom surface wall in order to prevent heat loss due to thermal radiation. Either of these methods are intended to improve the thermal insulation in the direction of the thickness of the thermal insulation layer.
However, in the case where the contents to be thermally insulated is stored and thermally insulated inside of this type of thermally insulated container with a lid placed on top, heat is transmitted along the wall of the inner container from the area which is in contact with the contents to the mouth portion of the inner container, and the thermal conduction loss from the mouth portion of the inner container due to heat loss occurring from this mouth portion of the inner container to the outside of the container cannot be ignored. In the same manner, it is necessary to consider the conduction loss from the peripheral portion of the bottom surface wall facing the storage space for the contents. The thermal conduction loss from the mouth portion of the inner container or the peripheral portion of the bottom surface wall cannot be reduced by such methods as making each thermal insulation layers thicker, or filling gases having low thermal conductivity into the thermal insulation layers, or disposing metallic foil on the surface of the side of the thermal insulation layer of the inner container or the bottom surface wall. In addition, especially in the case where the thermally insulated container is made to have a wide rim, the amount of thermal conduction loss from the mouth portion of the container increases, and in conventional models of the thermally insulated containers, there was a possibility that the heat retaining performance capabilities or the cold retaining performance capabilities were insufficient.